Question 885963
Let x= 1 whole pizza

A can eat x/10 of the pizza in 1 min
B can eat x/15 of the pizza in 1 min
Their grandfather can eat x/30 of the pizza in 1 min 

{{{x/10+x/15+x/30=1 min}}}
{{{(3x+2x+1x)/30=1 min}}}
{{{6x/30=1 min}}}
{{{x/5=1 min}}}
{{{x=5 mins}}}

x=300 seconds